The present invention concerns new dichroitic anthraquinone dyestuffs which are useful as components of liquid crystalline dielectrics for electro-optical indicator elements based on the guest-host effect.
For indicator elements based on liquid crystalline dielectrics, it is known to produce the electro-optical effect of the indication by incorporation of dichroitic or pleochroitic dyestuffs as a so-called "guest phase" in a liquid crystalline matrix which is the "host phase" (See G. H. Heilmeier et al., Molecular Crystals and Liquid Crystals, Volume 8 (1969), pages 293-304, for example, whose disclosure is incorporated by reference herein). The dyestuff molecules of the guest phase are oriented by the embedding host phase in which they are dissolved or distributed, corresponding to the applied electrical field. Because of their pleochroitic properties, depending upon the orientation, they exhibit a differing light absorption. Thus, in contrast to normal monochloritic dyestuffs, the amount of light absorbed by pleochroitic dyestuffs depends upon the orientation of their molecules to the electrical field vector of the incident light. Since application of an electric field to the dielectric distributed in a cell as a thin layer effects a reorientation of the nematic liquid crystals of the host phase, the pleochroitic dyestuffs embedded therein as a guest phase, are reorientated therewith. This leads to a change in their light absorption. The practical use of this technique, referred to in the literature as the guest-host effect, is described e.g., in published Federal Republic of Germany patent application No. 1,928,003, which is incorporated by reference herein. Examples of pleochroitic or dichroitic dyestuffs described there include indophenol blue, indigo derivatives, azo dyestuffs and the like. However, the contrast achieved with such ingredients is only sufficient for good readability when a polarization foil is used.
According to a more recent development, described, e.g., in published Federal Republic of Germany patent application No. 2,410,557, which is incorporated by reference herein, the polarizer can be omitted when, to the dielectric composed of nematic liquid crystals with positive dielectric anisotropy (as host phase) and pleochroitic dyestuff (as guest phase) incorporated therein, there is added a small amount, e.g., 0.1-15 wt %, of an optically-active material which brings about a helical structure (cholesteric structure) formation in the liquid crystal. Examples of such compositions, as well as the physical bases of the light absorption therein, are described in the work of D. L. White and G. N. Taylor entitled "New Absorptive Mode Reflective Liquid Crystal Display Device" (J. Appl. Physics, Volume 45 (1974), pages 4718-4723), which is incorporated by reference herein. Examples of the construction and operation of electro-optical indicator elements based on this effect are described in published Federal Republic of Germany patent application Nos. 2,639,675 and 2,658,568, which are incorporated by reference herein.
In the literature, liquid crystal indicator elements having cholesteric oriented host phase and dyestuff embedded therein as guest phase are frequently referred to as cholesteric guest-host displays. These CGH indicator elements have proved to be advantageous because, without polarization foils, they provide good indicator contrast and, in addition, have a greater brightness of the depicted image.
Choosing suitable dyestuffs for CGH indicator elements has proven to be very difficult. The dichroitic comparative values of these systems having a host phase and dyestuff embedded therein as guest phase, must be sufficiently great in order to impart to the indicator cell a sufficient brightness and a sufficient contrast ratio. The achievable contrast depends upon the degree of order S of the dyestuff in the liquid crystalline matrix. The degree of order can be expressed by the relationship EQU S=1/2.multidot.&lt;3 cos.sup.2 .theta.-1&gt;=(E.sub.II -E.sub.I)/(E.sub.II +2E.sub.I)
wherein .theta. is the angle between the molecular longitudinal axis of the dyestuff molecule and the optical axis of the liquid crystal; E.sub.II and E.sub.I are the values of the extinction of the indicator element when the measurements are carried out in the parallel orientation (E.sub.II) or the vertical orientation (E.sub.I) of the liquid crystal molecules.
A value which is easier to deal with in everyday practice and which is related to the degree of order S, thereby relating to the technical usefulness of a dyestuff-liquid crystal combination, is the so-called dichroitic ratio V, which represents the quotient of the above-defined extinction values: EQU V=(E.sub.II /E.sub.I)
In technically and industrially useful dyestuff-liquid crystal combinations, the value of V lies at or above 5.
In practice, the degree of order of a dyestuff depends, in the first place, upon its chemical structure, as well as upon the nature of the liquid crystalline matrix. A number of examples of this is described by R. I. Cox in "Molecular Crystals and Liquid Crystals," Volume 55 (1979), pages 1-33, which is incorporated by reference herein.
Apart from the degree of order or the dichroitic ratio, still other parameters are, however, important for the use of a dyestuff in CGH indicator elements. An absorption maximum of the dyestuff must lie in the range of the visible wavelengths, i.e., between 400 and 700 nm, and the dyestuff must be stable in the doped host phase up to a temperature of about 100.degree. C., against alternating voltages of up to 20 V, against radiation in the infra-red, visible and ultra-violet ranges and against the components of the liquid crystalline host phase. Furthermore, good solubility in the host phase and a high optical density are desirable.
However, the dyestuffs hitherto suggested for CGH indicator elements do not fulfill these requirements to a sufficient extent. In particular, the azo dyestuffs, which are preferred because of their dichroitic ratio and their absorption maxima and which are prevalently suggested for use in CGH indicator elements, have proven to be insufficiently stable against light or ultra-violet radiation (too low a photo-chemical stability). In some cases, they also are chemically unstable. On the other hand, the photochemical and chemical stability of anthraquinone dyestuffs, which is generally higher in comparison with azo dyestuffs, is known. Indeed, proposals have been made in the literature for the use of anthraquinone dyestuffs for CGH indicator elements. However, most of the previously suggested anthraquinone dyestuffs have too small a dichroitic ratio V to enable their technical and industrial use in CGH indicator elements.
In published British patent application No. 20 11 940, there are described substituted anthraquinone dyestuffs of the formula (A), ##STR3## R.sub.1 is an optionally ring-substituted aniline group attached via the nitrogen atom; R.sub.2 is H or OH; and R.sub.3 is H or, when R.sub.2 is H, also an optionally substituted aniline group. The dichroitic ratio of these anthraquinone dyestuffs, measured in commercially available cyanobiphenyl/cyanoterphenyl liquid crystal mixtures, lies between 3.8 and 8.4, whereby most values lie between 5 and 6. However, the solubility of these dyestuffs in the most conventional liquid crystalline host phases is reduced by the aniline groups. In particular, in the liquid crystalline base materials of the phenylcyclohexane class, used today to a wide extent, the solubility of the dyestuffs of formula (A) is too low to meet practical requirements.